Apparatus and method for generating protective atmospheres

ABSTRACT

A heat treatment furnace having a fuel-fired heating tube is provided with apparatus for producing a protective atmosphere for the furnace, which apparatus comprises a duct for conveying a part of the gas exhausted from the heating tube to a heat exchanger situated outside the furnace. The gas exhausted from the heating tube to the heat exchanger is cooled by the latter, such cooling causing water to condense from the gas. A further tube of larger diameter than the heating tube is arranged concentrically with the latter to provide an annular space along which the de-watered gas is passed into the furnace chamber. The apparatus further comprises a fuel inlet through which additional fuel can be added to the de-watered gas before the latter is passed into the annular space. While in the annular space, the mixture of de-watered gas and additional fuel is heated and undergoes an endothermic reaction to provide an endothermic atmosphere in the furnace. The apparatus may be operated without addition of fuel to the de-watered gas to provide an exothermic atmosphere.

United States Patent [191 Golding Apr. 16, 1974 APPARATUS AND METHOD FOR GENERATING PROTECTIVE ATMOSPHERES [75] Inventor: Ivor Sidney Lawrence Golding,

Sutton Coldfield, England [73] Assignee: Actric Limited, West Bramwich,

Staffordshire, England [22] Filed: Oct. 24, 1972 [21] Appl. No.: 300,355

[30] Foreign Application Priority Data Primary Examiner-John JfCamby Attorney, Agent, or Firm-Holman & Stern 57 ABSTRACT A heat treatment furnace having a fuel-fired heating tube is provided with apparatus for producing a protective atmosphere for the furnace, which apparatus comprises a duct for conveying a part of the gas exhausted from the heating tube to a heat exchanger situated outside the furnace. The gas exhausted from the heating tube to the heat exchanger is cooled by the latter, such cooling causing water to condense from the gas. A further tube of larger diameter than the heating tube is arranged concentrically with the latter to provide an annular space along which the dewatered gas is passed into the furnace chamber. The apparatus further comprises a fuel inlet through which aqqi iqaatfi e sen bee ded E9. tllgsryflsr s la: fore the latter is passed into the annular space. While in the annular space, the mixture of de-watered gas and additional fuel is heated and undergoes an endothermic reaction to provide an endothermic atmosphere in the furnace. The apparatus may be operated without addition of fuel to the de -watered gas to provide an exothermic atmosphere.

11 Claims, 1 Drawing Figure APPARATUS AND METHOD FOR GENERATING PROTECTIVE ATMOSPHERES SUMMARY OF THE INVENTION This invention relates to apparatus and to a method for the generation of protective atmospheres, for example atmospheres suitable for use in furnaces during the heat treatment of workpieces in a nonoxidising atmosphere.

The term heat treatment embraces tempering, normalising, sintering brazing, hardening, annealing, carburising and carbo-nitriding. When such operations are carried out it is generally necessary to avoid contact between the workpiece and oxygen, and in many cases the presence in the atmosphere surrounding the workpiece of other gases, for example water vapour, carbondioxide and hydrogen, must be avoided, or the concentrations of such gases must be carefully controlled.

Apparatus according to the invention may also be employed to provide a protective atmosphere for other purposes, for example the purging of air from storage containers into which inflammable material is to be introduced, the replacement in a storage container of an inflammable material removed therefrom and as in inert atmosphere in chemical process plant. Thus apparatus according to the invention could be employed to provide a protective atmosphere for the filling of the tanks of an oil tanker when the cargo is discharged therefrom.

The protective atmosphere for conventional heat treatment furnaces is generated in a generator separate from the furnace either by burning a hydrocarbon fuel in an approximately stochiometric quantity of air (an exothermic reaction), cooling the products of combustion to reduce the water content thereof and feeding this gas to the chamber of the furnace as a protective atmosphere (generally called an exothermic atmosphere), or alternatively by reacting fuel with a smaller quantity of air and supplying heat to the reactants, and then supplying the resultant gas to the furnace chamber as a protective atmosphere (generally termed an endothermic atmosphere). It will be appreciated that an exothermic atmosphere produced in this way will contain a small proportion, say 1 3 percent, of free oxygen and considerably larger proportions of water vapour and carbon dioxide. The proportions of these three gases in the endothermic atmosphere will be smaller than in the exothermic atmosphere, or even negligible. The generator associated with a particular furnace would be adapted either to produce a endothermic atmosphere or to produce an exothermic atmosphere, according to the intended use of the furnace.

One disadvantage of this conventional arrangement is that in the case of a generator adapted to produce an exothermic atmosphere, the heat which is relaesed in the exothermic reaction of fuel with air is wasted. Furthermore, cooling equipment must be provided specifically for removing this heat, the heat normally being transmitted ultimately to the ambient atmosphere or to water which is discarded.

In the case of a conventional generator adapted to produce an endothermic atmosphere, an additional quantity of fuel is burned in order to raise the temperature of the reacting fuel/air mixture.

One formof generator which has previously been proposed for producing a protective atmosphere for a heat treatment furnace is so arranged that an endothermic atmosphere is produced from the fuel in two stages, namely an exothermic stage and an endothermic stage, and that some of the heat released in the exothermic stage is applied to the reactants of the endothermic stage. However, this previous proposal enables a small proportion only of the heat of combustion of the fuel to be transferred to the furnace chamber, and a considerable proportion of the heat released in the exothermic reaction is still wasted.

In a previous attempt to reduce the capital cost of the furnace and generator, and to produce a more compact arrangement, it has been proposed to supply fuel gas mixed with a relatively small quantity of air to a packed tube situated within the furnace chamber. The fuel is partially oxidised in the tube and the gases exhausted therefrom are released directly into the furnace chamber as a protective atmosphere. l

The main disadvantage of this latter proposal is that in order to avoid an unacceptably high concentration of water vapour in the protective atmosphere, the proportion of air in the fuel/air mixture supplied to the tube must be small. In consequence of this, the range of composition of atmospheres which can be produced is very restricted. Also the rate of use of fuel necessary to produce the protective atmosphere at a specified rate is relatively high. It will be appreciated that combustion of the gases which form the protective atmosphere is completed only when these gases leave the furnace and burn in ambient air so that the heat released at this stage is not available for heating the furnace chamber. This proposal does not therefore make efficient use of the fuel.

It is an object of the present invention to provide a combination of heat treatment furnace and apparatus of the kind specified which is less expensive to manufacture, of more compact form, and less expensive to operate than the separategenerat-or and furnace hith erto generally employed.

According to one aspect of the invention there is provided in combination with or for combination with a heat treatment furnace, apparatus for producing a protective atmosphere and comprising at least one heating tube within which fuel is burned in air when the furnace is operating, such tube being disposed at least partly within the chamber of the furnace when the apparatus is in use so that heat can be radiated from the tube to workpieces in the chamber and/or to the internal surfaces of the chamber, a heat exchanger disposed outside the chamber, an outlet duct for conveying gases exhausted from the heating tube to the heat exchanger, the latter being arranged to cool such exhaust gases when the apparatus is in use, means for removing from the exhaust gas flow water which condenses therefrom, and a delivery duct for conveying the de-watered exhaust gases from the heat exchanger to the interior of the chamber as a protective atmosphere.

The term interior of the chamber means herein and in the appended claims the space within the chamber but outside the heating tube.

Apparatus according to the invention provides that heat released by reaction of the fuel with air to form the protective atmosphere is radiated into the chamber from the heating tube, and accordingly the running costs of the furnace are less than that of the conventional furnace and separate generator. Furthermore, the cost of construction of the furnace is less than that of the construction of a conventional furnace and generator of similarcapacity. A furnace including apparatus according to the invention may also be made in a more compact form than the conventional furnace and generator considered collectively.

The furnace may include additional heating elements in the form of either fuel fed radiant tubes or electrical heating elements.

Means may be provided for varying the relative quantities of fuel and air supplied to the heating tube to vary the composition of the protective atmosphere produced.

Preferably the heat exchanger is watencooled. The temperature to which the exhaust gases are cooled would depend on the required limit of water vapour concentrated in the de-watered exhaust gases.

We prefer to provide a passageway containing at least a part of said heating tube, the passageway communicating, when the heating tube is installed in a furnace, through an inlet with the delivery duct and through an outlet with the interior of the furnace chamber.

With this arrangement the de-watered gas passing to the interior of the chamber passes over the heating tube and is thereby heated before emerging through said outlet.

Preferably there is also provided means for introducing additional fuel into said delivery duct or into said passageway.

Such means enables the de-watered gas to be mixed with additional fuel and then heated in said passageway so that such additional fuel reacts with free oxygen, water vapour or carbon dioxide present to form an endothermic atmosphere.

Preferably the passageway is formed by a further tube surrounding a part of the heating tube.

This arrangement is advantageous since the annular passageway has a larger surface area than would a circular passageway of the same cross-sectional area. Contact between the gaseous mixture and surfaces of the heating tube and further tube promote chemical reactions between the constituents of the gaseous mixture. Either one or both of the heating tube and further tube may be formed of, or be coated with, a material which includes a catalyst to one or more of the required reactions. For example, both the heating tube and further tube may be formed of an alloy which includes nickel, this element being catalytically active in reactions which'are required to occur within the passageway. In the absence of a catalyst, mere contact between a somewhat rough surface presented by the further tube and heating tube would promote reaction between the constituents of the gaseous mixture.

The further tube may also be disposed within the furnace chamber, in which case heat can be supplied to the reactants of the endothermic reaction from both the heating tube and the furnace chamber or other heating elements therein.

Since the exhaust gas is de-watered before the endothermic reaction is carried out, the concentration of hydrogen present in the protective atmosphere within the chamber will be reduced considerably. This is important in certain treatments, for example the treatment of copper workpieces, where the presence of excessive hydrogen is objectionable. Furthermore, less heat is consumed in the endothermic reaction than would be the case if the water were not first removed,

By varying the rate at which additional fuel is introduced into the delivery duct or passageway, a wide range of atmosphere compositions can be produced.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing shows a diagrammatic part-sectional plan view of a heat treatment furnace chamber and apparatus for producing a protective atmosphere therefor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The furnace illustrated in the accompanying drawing comprises a heating chamber 10 through which workpieces are conveyed and within which heat is required to be supplied to the workpieces. The construction of the chamber and of the means for conveying the workpieces therein may be substantially conventional. Thus, there may be provided a shaker hearth, walking beam, roller hearth, mesh belt, cast link, rotary hearth or like conveyor for the workpieces. The conveyor may be arranged to discharge workpieces through an aperture in a floor of the furnace into a quench tank. Alternatively, the furnace may be adapted for the treatment of workpieces in batches.

Combined with the furnace there is apparatus for producing and delivering to the chamber 10 a protective atmosphere. This apparatus includes a heating tube 1 1 conveniently mounted horizontally within the cham- .ber 10, an end portion of the tube projecting through an end wall 12 of the chamber to a burner 13. A gaseous hydrocarbon fuel and air are introduced into the tube 1 1 through the burner so that the fuel burns within the tube and the latter is heated and radiates heat to the interior of the chamber 10 and to workpieces therein. Separately controllable valves would be provided for controlling the supply of fuel and air to the burner 13 so that the rate of supply of each of these can be varied independently. These valves would be so set that the fuel/air ratio is sufficiently high to ensure that all of the free oxygen in the air is consumed.

An outlet duct 14 communicates with the interior of the heating tube 11 at the end thereof remote from the burner 13. The outlet duct may be sealed to the heating tube so that all of the products of the combustion occurring therein pass through the outlet duct. Alternatively the outlet duct may, as shown, be arranged to receive only a part of the gases exhausted from the heating tube 11, the remainder of these exhaust gases passing on to a further part of the furnace through apertures (not shown) in the tube.

The outlet duct 14 is arranged to convey gases exhausted from the heating tube 11 to a heat exchanger 15 situated outside the chamber 10. The heat exchanger may be water-cooled. Since the fuel supplied to the burner 13 is a hydro-carbon fuel gases exhausted from the heating tube 11 will contain a considerable proportion of water vapour. When these gases pass through the heat exchanger and are cooled, the greater part of this water vapour will condense to the liquid state. Means is provided for separating this liquid water from the gas flow, and such means comprises a well 16 into which the water can drain. As shown, a tap may be provided for periodically removing water from the well. Alternatively, the well may be provided with a known form of overflow which seals the interior of the well against ingress of ambient air.

A blower 17 is provided to draw the de-watered exhaust gases from the heat exchanger and pass same to a delivery duct 18. The delivery duct is provided with fuel inlet means 19 through which additional fuel can be introduced into the exhaust gas flow. A further valve is provided for controlling the rate of addition of fuel according to the required composition of the furnace atmosphere.

A further tube 20 having larger cross-sectional dimensions than the heating tube 11 is mounted concentrically with the latter so as to define an annular passageway 21 between the walls of the two tubes. The further tube 20 extends over a part only of the length of the heating tube 11 so that the passageway 21 extends from a position adjacent to the burner 13, which end of the passageway is closed, to a position within the chamber 10, this end of the passageway being open to the chamber and forming an outlet 22 of the passageway. The delivery duct 18 communicates with the passageway 21 through an inlet 23 adjacent to the closed end of the passageway.

The furnace would normally include further heating elements (one of which is shown) which may be of conventional form, for example electrically energisable heating elements or radiant fuel-buming tubes. These heating elements may be arranged in a conventional manner within the chamber 10.

When the furnace is operating, air and fuel are fed at predetermined rates to the burner 13 and react together within the heating tube 11. This reaction is an exothermic one so that the tube 11 becomes heated and radiates heat outw'ardly. Some of this heat is radia-ted directly into the furnace chamber to heat the walls of the latter and workpieces therein. A further part of this heat would be radiated to the further tube so that this latter tube also becomes heated.

A part of the products of this exothermic reaction are exhausted from the heating tube 11 through the outlet duct 14 to the heat exchanger 15. The main constituents of the exhaust gases would normally be nitrogen, water vapour and carbon-dioxide. When these gases are cooled, condensation of water vapour to liquid water occurs and the liquid water collects in the well 16. The amount of water vapour remaining in the dewatered exhaust gases will depend upon the temperature to whichthe gases have been cooled. This temperature would be selectedaccording to the required composition of the furnace atmosphere.

The de-watered exhaust gases may be passed to the furnace chamber 10 through the annular passageway 20 without addition of further fuel, as an exothermic atmosphere.

If an endothermic atmosphere is required in the furnace chamber 10, a small quantity of additional fuel would be introduced at. the fuel inlet 19. The mixture of additional fuel and de-watered exhaust gases would then pass over the hot surfaces of the heating tube 11 and further tube 20 which form the boundaries of the passageway 21". An endothermic reaction between the additional fuel and carbon-dioxide, and possibly also between the additional fuel and water vapour, takes place within the passageway 21 so that an endothermic atmosphere having low concentrations of carbondioxide and water vapour would enter the furnace chamber. One or both of the tubes 11 and 20 may be formed of an alloy which includes nickel and is therefore catalytically active with respect to the endothermic reaction.

When the apparatus is supplying an exothermic atmosphere to the furnace chamber, a considerable proportion of the heat released by reaction between the fuel and air within the heating tube is transmitted from the latter to the chamber and any workpieces therein. Since the gases exhausted through the outlet duct 14 will have a temperature somewhat higher than that of the interior of the chamber 10, a significant quantity of heat is unavoidably carried by the exhaust gases from the chamber to the heat exchanger. The present arrangement does, however, permit operation of the furnace with a lower heat output from such further heating elements as are provided that would be the case if none of the heat of this exothermic reaction were applied to the furnace chamber. Thus generally the number of further heating elements can be reduced, this contributing to a reduction in the capital cost of the furnace and in the running cost thereof.

When the apparatus is producing an endothermic atmosphere, some of the heat released by the exothermic reaction within the heating tube [1 is absorbed by the endothermic reaction occurring within the passageway 21. Generally this endothermic reaction would absorb only a minor part of the heat which is released by the exothermic reaction. If there is a requirement for a protective atmosphere having a composition such that the endothermic reaction will absorb heat at such a rate that heat supplied from that part of the heating tube 1 l disposed within the passageway 21 is insufficient to maintain the temperature of the reactants at a sufficiently high value, further heat can be supplied to the reactants within the passageway 20 through the walls of the further tube 20 from such further heating elements as are present within the chamber. It will be noted that the removal of water vapour from the gases before the endothermic reaction takes place reduces the amount of heat required to complete the endothermic reaction, as compared with the amount of heat which would be required if no water vapour were removed. A further "advantage of removing water vapour from the gases is that a required carbon potential of the protective atmosphere can be achieved with a relatively low rate of addition of fuel at the fuel inlet 19.

The furnace, including the apparatus for producing the protective atmosphere, shown in the accompanying drawing is considerably more compact than the combination of conventional furnace and generator. Furthermore, apparatus according to the present invention can be constructed considerably more cheaply than can a separate generator capable of producing protective atmosphere at a similar rate.

The apparatus for producing a protective atmosphere described herein may be utilised in a heating appliance other than a heat treatment furnace. For example, the apparatus may be employed in a boiler.

The boiler includes a heating chamber for containing a body of water to be heated. The boiler further includes one or more heating elements disposed either within the heating chamber for direct contact with the body of water, or outside the heating chamber, in which case heat is transferred from the heating elements to a wall of the heating chamber by radiation and possibly by convection, and then by conduction through the wall of the heating chamber to the body of water.

The heating element comprises first and second passageways, which may be formed by concentric tubes. A burner is provided to deliver fuel and air to a first of these passageways to burn therein, and an outlet duct leads from this first passageway to a heat exchanger screened from the heating element and disposed outside the heating chamber. A delivery duct is provided to convey gases from the heat exchanger to the second passageway and means is provided for introducing additional fuel into this delivery duct or directly into the second passageway.

When the boiler is operating, heat released by combustion of the fuel within the first passageway is transferred to the second passageway and also to the body of water within the heating chamber. Gases exhausted from the first passageway are cooled in the heat exchanger to condense at least some of the water therefrom, and the de-watered gases are caused to react with additional fuel in the heated second passageway to form the protective atmosphere. This atmosphere would be conveyed by a further duct from the second passageway to the point of use.

The heat exchanger could be arranged to transfer heat from the gases exhausted from the first passageway to water which subsequently passes into the heating chamber to be heated further therein. Further burners or heating elements could be provided, the exhaust gases therefrom not being recovered as a protective atmosphere. I

The heat exchanger may be arranged to provide either direct or indirect contact between a liquid coolant, for example water, and the exhaust gases. There may be further provided, in association with said heat exchanger or a further heat exchanger, means for compressing the exhaust gases before passage through the heat exchanger and orifice means through which the gases expand after passing through the heat exchanger, whereby the exhaust gases will be further cooled by adiabatic expansion. In this way, the gases could readily be cooled below C to reduce the concentration of water vapour to a very low value.

I claim:

1. The combination comprising a heat treatment furnace having a treatment chamber and apparatus for producing a protective atmosphere for such chamber wherein the apparatus comprises:

a heating tube disposed at least partly within the treatment chamber,

means for supplying fuel and air to the heating tube,

a heat exchanger disposed outside the treatment chamber an outlet duct for conveying to the heat exchanger a stream of gas exhausted from the heating tube,

means for removing from said gas stream flowing through the heat exchanger water which condenses therefrom,

a delivery duct for conveying said gas stream from the heat exchanger to the treatment chamber,

and a passageway which contains at least a part of said heating tube, the passageway communicating through an inlet with the delivery duct and through an outlet with the interior of the treatment chamber,

whereby fuel supplied to the heating tube is burnt therein to cause the heating tube to radiate heat which is received by the internal surfaces of the treatment chamber, products of this combustion are cooled in the heat exchanger to condense water therefrom, and the de-watered gas stream is heated in said passageway and supplied to the treatment chamber as a protective atmosphere.

2. The combination comprising a heat treatment furnace having a treatment chamber and apparatus for producing a protective atmosphere for such chamber wherein the apparatus comprises:

a heating tube disposed at least partly within the treatment chamber,

means for supplying fuel and air to the heating tube,

a heat exchanger disposed outside the treatment chamber,

an outlet duct for conveying to the heat exchanger a stream of gas exhausted from the heating tube,

means for removing from said gas stream flowing through the heat exchanger water which condenses therefrom,

a delivery duct for conveying said gas stream from the heat exchanger to the treatment chamber,

and means for introducing additional fuel into said delivery duct,

whereby fuel supplied to the heating tube is burnt therein to cause the heating tube to radiate heat to the internal surfaces of the treatment chamber, products of this combustion are cooled in the heat exchanger to condense water therefrom, and the de-watered gas stream is supplied to the treatment chamber as a protective atmosphere.

3. The combination claimed in claim 2 wherein there is provided a passageway which contains at least a part of said heating tube, the passageway communicating through an inlet with the delivery duct and through an outlet with the interior of the treatment chamber.

4. The combination claimed in claim 3 wherein said passageway is formed by a further tube surrounding a part of the heating tube.

5. The combination claimed in claim 4 wherein said further tube is disposed at least partly within the treatment chamber.

6. Apparatus for producing a protective atmosphere and for supplying heat to a body which is required to be heated, such apparatus comprising:

a heating chamber for containing the body which is required to be heated,

a heating element for supplying heat to be imparted to the body, the heating element being arranged to supply heat at least by radiation to at least the walls of the heating chamber, and comprising first and second passageways separated by a dividing wall,

means for supplying fuel and air to the first passageway,

a heat exchanger disposed outside the heating chamber and screened from the heating element,

an outlet duct for conveying to the heat exchanger a stream of gas exhausted from the first passageway, I

means for removing from said gas stream flowing through the heat exchanger water which condenses therefrom,

a delivery duct for conveying the stream of dewatered gas from the heat exchanger to said second passageway,

and means for introducing additional fuel into said delivery duct to mix with the de-watered gas stream therein,

whereby the mixture of additional fuel and dewatered gas stream within the second passageway will receive heat from the first passageway and will react to produce the protective atmosphere.

7. A method of producing a protective atmosphere and simultaneously heating a body, wherein:

fuel is burned in air within a first passageway,

a stream of gas consisting of products of combustion is exhausted from the first passageway,

the water content of said gas stream is reduced,

additional fuel is mixed with the de-watered gas stream,

the mixture of fuel and dewatered gas stream is passed along the second passageway,

and heat is transferred from the first passageway to both said body and said mixture in the second passageway, thereby causing further reaction to occur within the second passageway to form the protective atmosphere.

8. A method of producing a protective atmosphere for use in a treatment chamber of a heat treatment furnace, wherein:

a fuel is burned in a heating tube which lies at least partly within the treatment chamber,

a stream of gas consisting of products of combustion is exhausted from the heating tube to a position outside the treatment chamber,

the water content of said stream of gas is reduced whilst outside the treatment chamber,

the de-watered gas stream is heated by passage over the surface of the heating tube and the heated dewatered gas stream is delivered into the treatment chamber as a protective atmosphere.

9. The method claimed in claim 8 wherein additional fuel is introduced into the de-watered gas stream before the latter passes over the surface of the heating tube. I 10. The method claimed in claim 8 wherein the prod ucts of combustion exhausted from the heating tube are divided into two parts, a first part is discarded and a second part is subjected to treatment to reduce its water content.

11. The method claimed in claim 8 wherein the protective atmosphere is discarded after passage through the treatment chamber and is maintained separate from the burning fuel. 

1. The combination comprising a heat treatment furnace having a treatment chamber and apparatus for producing a protective atmosphere for such chamber wherein the apparatus comprises: a heating tube dIsposed at least partly within the treatment chamber, means for supplying fuel and air to the heating tube, a heat exchanger disposed outside the treatment chamber an outlet duct for conveying to the heat exchanger a stream of gas exhausted from the heating tube, means for removing from said gas stream flowing through the heat exchanger water which condenses therefrom, a delivery duct for conveying said gas stream from the heat exchanger to the treatment chamber, and a passageway which contains at least a part of said heating tube, the passageway communicating through an inlet with the delivery duct and through an outlet with the interior of the treatment chamber, whereby fuel supplied to the heating tube is burnt therein to cause the heating tube to radiate heat which is received by the internal surfaces of the treatment chamber, products of this combustion are cooled in the heat exchanger to condense water therefrom, and the de-watered gas stream is heated in said passageway and supplied to the treatment chamber as a protective atmosphere.
 2. The combination comprising a heat treatment furnace having a treatment chamber and apparatus for producing a protective atmosphere for such chamber wherein the apparatus comprises: a heating tube disposed at least partly within the treatment chamber, means for supplying fuel and air to the heating tube, a heat exchanger disposed outside the treatment chamber, an outlet duct for conveying to the heat exchanger a stream of gas exhausted from the heating tube, means for removing from said gas stream flowing through the heat exchanger water which condenses therefrom, a delivery duct for conveying said gas stream from the heat exchanger to the treatment chamber, and means for introducing additional fuel into said delivery duct, whereby fuel supplied to the heating tube is burnt therein to cause the heating tube to radiate heat to the internal surfaces of the treatment chamber, products of this combustion are cooled in the heat exchanger to condense water therefrom, and the de-watered gas stream is supplied to the treatment chamber as a protective atmosphere.
 3. The combination claimed in claim 2 wherein there is provided a passageway which contains at least a part of said heating tube, the passageway communicating through an inlet with the delivery duct and through an outlet with the interior of the treatment chamber.
 4. The combination claimed in claim 3 wherein said passageway is formed by a further tube surrounding a part of the heating tube.
 5. The combination claimed in claim 4 wherein said further tube is disposed at least partly within the treatment chamber.
 6. Apparatus for producing a protective atmosphere and for supplying heat to a body which is required to be heated, such apparatus comprising: a heating chamber for containing the body which is required to be heated, a heating element for supplying heat to be imparted to the body, the heating element being arranged to supply heat at least by radiation to at least the walls of the heating chamber, and comprising first and second passageways separated by a dividing wall, means for supplying fuel and air to the first passageway, a heat exchanger disposed outside the heating chamber and screened from the heating element, an outlet duct for conveying to the heat exchanger a stream of gas exhausted from the first passageway, means for removing from said gas stream flowing through the heat exchanger water which condenses therefrom, a delivery duct for conveying the stream of dewatered gas from the heat exchanger to said second passageway, and means for introducing additional fuel into said delivery duct to mix with the de-watered gas stream therein, whereby the mixture of additional fuel and de-watered gas stream within the second passageway will receive heat from the first passageway and will react to produce the protective atmosphere.
 7. A mEthod of producing a protective atmosphere and simultaneously heating a body, wherein: fuel is burned in air within a first passageway, a stream of gas consisting of products of combustion is exhausted from the first passageway, the water content of said gas stream is reduced, additional fuel is mixed with the de-watered gas stream, the mixture of fuel and de-watered gas stream is passed along the second passageway, and heat is transferred from the first passageway to both said body and said mixture in the second passageway, thereby causing further reaction to occur within the second passageway to form the protective atmosphere.
 8. A method of producing a protective atmosphere for use in a treatment chamber of a heat treatment furnace, wherein: a fuel is burned in a heating tube which lies at least partly within the treatment chamber, a stream of gas consisting of products of combustion is exhausted from the heating tube to a position outside the treatment chamber, the water content of said stream of gas is reduced whilst outside the treatment chamber, the de-watered gas stream is heated by passage over the surface of the heating tube and the heated de-watered gas stream is delivered into the treatment chamber as a protective atmosphere.
 9. The method claimed in claim 8 wherein additional fuel is introduced into the de-watered gas stream before the latter passes over the surface of the heating tube.
 10. The method claimed in claim 8 wherein the products of combustion exhausted from the heating tube are divided into two parts, a first part is discarded and a second part is subjected to treatment to reduce its water content.
 11. The method claimed in claim 8 wherein the protective atmosphere is discarded after passage through the treatment chamber and is maintained separate from the burning fuel. 